* some pam_sys constants are u32 on FreeBSD instead of i32.
* The CONV_AGAIN and INCOMPLETE flags don't exist on FreeBSD, and weren't used
  in this crate.  Remove them.
* The Fail_Delay, XDisplay, XAuthData, and AuthTok_Type pam items types don't
  exist on FreeBSD, and aren't used in this crate, so remove them.
* The Disallow_Null_AuthTok and PrelimCheck pam flags have the same value on
  FreeBSD as Establish_Cred.  But they aren't actually used, so just remove
  them.
* The Update_AuthTok pam flag has the same value on FreeBSD as Establish_Cred.
  But it isn't actually used, so just remove it.
* The Change_Expired_AuthTok pam flag has the same value on FreeBSD as
  Reinitialize_Cred.  But it isn't actually used, so just remove it.
* Fix the drop_env_list function, which never worked (or even compiled) on
  non-Linux.

Fixes 1wilkens#42